Device for purifying a mercury-containing flue gas

ABSTRACT

Device for purifying a mercury-containing flue gas, at least comprising dust removing means for removing dust from a flue gas, and a quantity of a regenerable amalgamable material for removing mercury from a flue gas, which device is adapted for removal therefrom in amalgamated state of the quantity of amalgamable material for the purpose of regenerating this material, wherein the material is provided in a manner such that it can be regenerated in amalgamated state outside the device.

BACKGROUND Field of the Invention

The invention relates to a device for purifying a mercury-containingflue gas, at least comprising dust removing means for removing dust froma flue gas, and a quantity of a regenerable amalgamable material forremoving mercury from a flue gas.

From the international patent publication WO 92/16284 is known a filterfor removing mercury (Hg) from flue gas, which filter comprises ahousing with an inlet opening and an outlet opening for gases forpurifying and a filter body. The filter body consists of a large numberof layers of a thin metal foil covered with a gold layer, between whichlayers channels are formed for guiding gases for purifying along theselayers. In the known filter electrical heating wires or other means areprovided for heating said metal foil and the gold layer depositedthereon to a temperature above 20° C. in order to regenerate the filterin situ. During regeneration of the filter air or another gas ispreferably guided through said channels.

The known filter is particularly intended for use in combination with acremation furnace to remove mercury originating from dental fillings.Due to the heating installation required for regenerating the gold foiland the cooling device inherently associated therewith which enablescondensation of the mercury coming from the filter and released duringregeneration, the filter is expensive to purchase and its complexitymakes operation vulnerable. Replacement of the filter body requiresdisassembly of the electrical heating cables and the filter housing,which may necessitate a prolonged and usually unexpected and thereforeundesirable interruption in operations.

SUMMARY OF THE INVENTION

The object of the invention is to provide an operationally reliablepurifying device for removing mercury from flue gas, which isparticularly suitable for use in combination with a cremation furnace,which can be manufactured and operated at relatively low cost and whichcan be regenerated in simple manner without expensive provisions beingrequired for this purpose or normal operations having to be discontinuedfor a protracted period.

These objectives are achieved, and other advantages obtained, with apurifying device of the type stated in the preamble, which device isadapted according to the invention for removal therefrom in amalgamatedstate of the quantity of amalgamable material for the purpose ofregenerating this material, wherein the material is provided in a mannersuch that it can be regenerated in amalgamated state outside the device.

Amalgamable material is here understood to mean any material which iscapable of alloying with mercury or of binding with mercury byadsorption; amalgamated state is understood to mean a state in which thematerial is alloyed with mercury or has bound with mercury byadsorption.

In an embodiment of a purifying device according to the invention thequantity of amalgamable material is a quantity of amalgamable metalwhich is accommodated in removable manner in a filter housing which isadapted to guide a mercury-containing flue gas through this filterhousing and in contact with this metal.

The amalgamable metal is for instance metal selected from the groupcomprising copper (Cu), silver (Ag) and gold (Au).

In an advantageous embodiment the amalgamable metal is copper (Cu) withsulphur (S) bound thereto, this being obtained for instance from areaction of copper (Cu) with solid sulphur (S) at a temperature of about104° C. By binding S to Cu the surface of Cu available for amalgamationis enlarged, while the capacity for binding mercury is further increasedby the S, which forms the compound mercury sulphide (HgS) with mercury.

It has been found that mercury-containing copper with sulphur boundthereto can be regenerated in simple manner in a vacuum oven at atemperature of about 550° C., wherein formed HgS decomposes into Hg andS, and Hg which has been taken up into Cu with the formation of a CuHgalloy is released.

In a subsequent embodiment of a purifying device according to theinvention the quantity of amalgamable material is a quantity ofinorganic amalgamable material which is accommodated in removable mannerin a filter housing which is adapted to guide a mercury-containing fluegas through this filter housing and in contact with this material.

The quantity of inorganic amalgamable material is poured for instance inpowder or granule form into a container with a gas-permeable bottomwhich fits into a filter housing such that flue gas guided through thishousing is guided via the bottom of the container through the quantityof amalgamable material, wherein mercury in the flue gas is bound bythis material.

In an advantageous embodiment of a purifying device according to theinvention the inorganic amalgamable material is a mixture of titaniumdioxide (TiO₂) and divanadium pentoxide (V₂O₅), wherein the quantity ofV₂O₅ in the mixture lies for instance in the region of 4-7% by weight,the TiO₂ forms a carrier for the V₂O₅ and/or the carrier of TiO₂ and theV₂O₅ carried thereby is provided by particles with a nominal diameter ofabout 0.8 mm.

In a device according to the invention the quantity of amalgamablematerial is accommodated for instance in a laminar flow reactor (LFR)for removing mercury from a mercury-containing flue gas guided in alaminar flow through this reactor in contact with this material.

In a particularly advantageous embodiment the quantity of amalgamablematerial in a device according to the invention is accommodated in aradial flow reactor (RFR) for removing mercury from a mercury-containingflue gas guided in a radial flow through this reactor in contact withthis material.

In a radial flow reactor in a device according to the invention thequantity of amalgamable material is provided in a cylindrical reactorvessel between two internal cylindrical walls, and a flue gas is guidedin radial direction from the outer wall through the quantity ofamalgamable material to the axis of the cylindrical reactor vessel,wherein a pressure gradient is created over the quantity of amalgamablematerial which enhances the amalgamation process.

In a very advantageous embodiment of a device according to the inventionthe dust removing means comprise a cyclone or an electro-filter placedin a conduit system for a mercury-containing flue gas flow upstream inthe device relative to the quantity of amalgamable material.

Use of a cyclone, which in this respect should also be understood tomean a multi-cyclone and a rotating particle separator, or anelectro-filter makes it possible to filter out dust particles from aflue gas at a relatively short distance from the relevant furnace at atemperature which is so high that the flue gas is not saturated withmercury, and all the mercury present in the gas is therefore carriedalong further downstream in said conduit system to the amalgamablematerial present in the device.

The invention will be elucidated hereinbelow on the basis of anembodiment and with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing FIG. 1 shows a diagram of an embodiment of a purifyingdevice according to the invention in a configuration in which the deviceis used for selective purifying of flue gas from a cremation furnace.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows schematically a flue gas purifying device which isconnected to the chimney 2 of a cremation furnace (not shown) which isclosable with a control valve 1. The flue gas purifying device iscomposed successively of a cyclone 3, a fine dust filter 4, the outletopening 15 of which is in contact with an amalgam filter 6 and anactivated carbon filter 7 via a conduit 13 in which a beat exchanger 10and a fan 5 are accommodated. The figure further shows a collectingtube/dust tube 12 under cyclone 3 for collecting dust, and an inletopening 8 for cooling air closable by a control valve 9. The operationof the purifying device is as follows. It is known that during acremation mercury from dental fillings is released about 10 to 50minutes after the beginning of the cremation process for a periodlasting about 30 minutes. The release of mercury is detected in thedevice using a suitable mercury sensor (not shown), whereupon controlvalve 1 is closed using a per se known circuit and the flue gas isguided via chimney 2 through cyclone 3, where hot flue gas purified ofcoarse dust particles is guided by; means of fan 5 via a fine dustfilter 4 through a conduit 13 along a heat exchanger 10 to the amalgamfilter 6. The temperature of the flue gas in amalgam filter 6 is broughtto about 150° C. using heat exchanger 10 and optionally by drawing incold ambient air via inlet opening 8. With an amalgam filter 6 in whichthe amalgamable material is for instance copper, silver or gold, about90% of the quantity of mercury present in the flue gas is bound. Theremaining amount of mercury is absorbed in the activated carbon filter7, which is impregnated with for instance sulfur (S) or potassium iodide(KI). In addition to the mercury remnant, dioxins and furans present inthe flue gas are also bound in activated carbon filter 7. Fine dustfilter 4 is periodically cleaned when the device is not in use byblowing compressed air therethrough in counter flow as according to thedirection of arrow 11, wherein dust which has been blown loses iscollected in dust tube 12 of cyclone 3. The filter body of amalgamfilter 6 is periodically replaced in fast and simple manner by a new orregenerated filter body. For this purpose the amalgam filter 6 has amodular assembly. A filter body with bound mercury is heated in an ovenat a temperature of about 850° C., wherein mercury is released which iscarried along by a gas flow and collected in a cooler where it condensesat a temperature of about −30° C. The amount off mercury captured in afilter amounts at a rough estimate to about 740 grams after 250cremations which in normal circumstances are carried out in acrematorium in a period of six months.

As an alternative to an amalgam filter 6 with copper, silver or gold, alaminar flow reactor (LFR) of a modular type can be used, wherein aneasily exchangeable filter body is filled with particles of a porousstructure and a nominal diameter of about 0.8 mm, composed of titaniumdioxide (TiO₂) and 4-7% by weight of divanadium pentoxide (V₂O₅),wherein the TiO₂ forms a carrier for the V₂O₅ present in a quantity of4-7% by weight. Such a material is commercially available from CRICatalyst Company U.K. Ltd., and is supplied under the name “CatalystS096” as catalyst for reducing nitrogen oxides (NO_(x)). Use of thismaterial in a device according to the invention offers the advantagethat nitrogen oxides are also removed from a flue gas, which is a legalrequirement in some countries for flue gas purifying installations forcremation furnaces.

It is noted that the purifying device according to the invention isparticularly suitable for use in combination with a cremation furnace,but that use of the device is not limited thereto.

The following experiments demonstrate the action of said compositionV₂O₅/TiO₂ (“Catalyst S096”) as mercury-assimilating material.

EXAMPLE 1

In a through-flow tube with a diameter of 1 cm mercury vapour-containingair with an Hg concentration of more than 2000 μg/m³ was guided at atemperature of 22° C., an air pressure of 1013 mBar and a relativehumidity of 50% for 0.5 hour through a sample of 5 g V₂O₅/TiO₂ “CatalystS096” and subsequently via a plastic Tygon hose with dust filter to a UVmercury vapour meter, The measured Hg concentration after guiding of theair through the sample of V₂O₅/TiO₂ amounted to 25 μg/m³, whichcorresponds to a mercury reduction of more than 98.75%.

EXAMPLE 2

The experiment described under example 1 was repeated at a temperatureof 100° C. under otherwise identical conditions. The measured Hgconcentration after guiding of the air through the sample of V₂O₅/TiO₂amounted to 250 μg/m³, which corresponds to a mercury reduction of morethan 87.50%.

EXAMPLE 3

The experiment described under example 1 was repeated under whollyidentical conditions. The measured Hg concentration after guiding of theair through the sample of V₂O₅/TiO₂ amounted to 5 μg/m³, whichcorresponds to a mercury reduction of more than 99.75%.

EXAMPLE 4

The experiment described under example 1 was repeated wherein themercury vapour-containing air was guided through the sample for 10 hoursunder otherwise identical conditions. The measured Hg concentrationafter guiding of the air through the sample of V₂/Ti₂ amounted to 1μg/m³, which corresponds to a mercury reduction of more than 99.95%.

COMPARATIVE EXAMPLE 1

The experiment described under example 1 was repeated wherein the sampleof V₂O₅/TiO₂ was replaced by a rod-like sample of 6 g active carbon witha length of 3 mm under otherwise identical conditions. The measured Hgconcentration after guiding of the air through the sample amounted to 5g/m³, which corresponds to a mercury reduction of more than 99.75%.

COMPARATIVE EXAMPLE 2

The experiment described under example 1 was repeated wherein the sampleof V₂O₅/TiO₂ was replaced by a 3 mm-long rod-like sample of 6 g ofactive carbon which was impregnated with 5% potassium iodide (KI) underotherwise identical conditions. The Hg concentration after guiding ofthe air through the sample was not measurable, which corresponds with amercury reduction of about 100%.

COMPARATIVE EXAMPLE 3

The experiment described under comparative example 2 was repeatedwherein the mercury vapour-containing air was guided over the sample for200 hours under otherwise identical conditions. The Hg concentrationafter guiding of the air through the sample was not measurable, whichcorresponds with a mercury reduction of about 100%.

COMPARATIVE EXAMPLE 4

The experiment described under example 1 was repeated wherein the sampleof V₂O₅/TiO₂ was replaced by a sample of 7 g silver wool with a wirethickness of 0.05 mm under otherwise identical conditions. The measuredHg concentration after guiding of the air through the sample amounted to1 μg/m³, which corresponds to a mercury reduction of more than 99.95%.

COMPARATIVE EXAMPLE 5

The experiment described under comparative example 4 was repeatedwherein the mercury vapour-containing air was guided through the samplefor 10 hours under otherwise identical conditions. The measured Hgconcentration after guiding of the air through the sample amounted to700 μg/m³, which corresponds to a mercury reduction of more than 62.50%.

The results of the above described experiments are collected in table 1.

TABLE 1 Results mercury reduction from mercury vapour- containing airDuration Conc. Conc. Quan- of of Hg of Hg tity contact before after Hgof sample Hg contact contact reduc- sample vapour sample sample tionSample (g) (hours) (μg/m³) (μg/m³) (%) Ex- ample 1 V₂O₅/TiO₂ 5 0.5 >200025 >98.75 “Catalyst S096” 2 V₂O₅/TiO₂ 5 0.5* >2000 250 >87.50 “CatalystS096” 3 V₂O₅/TiO₂ 5 0.5 >2000 5 >99.75 “Catalyst S096” 4 V₂O₅/TiO₂ 510 >2000 1 >99.95 “Catalyst S096” Compa- rative example 1 Active 60.5 >2000 5 >99.75 carbon 2 Active 6 0.5 >2000 0 100 carbon with 5% KI 3Active 6 200 >2000 0 100 carbon with 5% KI 4 Silver wool 7 0.5 >20001 >99.95 5 Silver wool 7 10 >2000 700 >62.50 *at a temperature of 100°C.

What is claimed is:
 1. A device for purifying a mercury-containing fluegas, comprising: dust removing means (3, 4, 12) for removing dust fromthe flue gas, and a quantity of a regenerable amalgamable material in afilter housing (6) that is configured to guide the mercury-containingflue gas through this filter housing (6) and in contact with thismaterial for removing mercury from the flue gas, characterized in thatthe quantity of amalgamable material is provided in a manner such thatin amalgamated state it is removable from the filter housing (6) for thepurpose of regenerating this material outside the device, theamalgamable material being a composition of titanium dioxide (TiO₂) anddivanadium pentoxide (V₂O₅).
 2. The device as claimed in claim 1,characterized in that the quantity of V₂O₅ in the composition lies inthe region of 4-7% by weight.
 3. The device as claimed in claim 1,characterized in that the TiO₂ forms a carrier for the V₂O₅.
 4. Thedevice as claimed in claim 3, characterized in that the carrier of TiO₂and the V₂O₅ carried thereby is provided by particles with a nominaldiameter of about 0.8 mm.
 5. The device as claimed in claim 1,characterized in that the quantity of amalgamable material isaccommodated in a laminar flow reactor (LFR) within the filter housingfor removing mercury from the mercury-containing flue gas which isguided in a laminar flow through this reactor in contact with thismaterial.
 6. The device as claimed in claim 1, characterized in that thequantity of amalagamable material is accommodated in a radial flowreactor (RFR) within the filter housing for removing mercury from themercury-containing flue gas guided in a radial flow through this reactorin contact with this material.
 7. The device as claimed in any of theforegoing claims, characterized in that the dust removing means comprisea cyclone (3) or an electro-filter placed in a conduit system for themercury-containing flue gas flow upstream in the device relative to thequantity of amalgamable material.